Sensitometers are employed in the manufacture of photosensitive media, e.g. photographic and radiographic film and paper, for process control to ensure product uniformity. A sensitometer is employed to expose test samples of the photosensitive product in production to a successive series of controlled optical power levels (optical power output over time) in ascending or descending order which result in a "step wedge" exposure of the test sample of the type depicted in FIG. 1. The appearance of the step wedge W on the product sample after developing is a progression of gray scale exposure steps or bands 1-20 ranging from a "black" level at one end band, arbitrarily designated "1" in FIG. 1, to a "white" level at the other end band, arbitrarily designated "20" in FIG. 1. Typical exemplary density values are shown numerically alongside the step wedge bands 1-20, although they are not exposed with the step wedge W. The "grayness" of each band may not be distinguishable by the naked eye, but can be can be determined and compared to a standard for the product type by using a densitometer. If a measurable deviation is found in any or all of the bands, then an appropriate change in the production can be undertaken to correct the photosensitivity of the product. In large scale production of such photosensitive product, it is necessary to successively expose step wedges of the type depicted in FIG. 1 on samples drawn from every few thousand square feet of product and complete the densitometry comparison to maintain process accuracy and meet product quality standards. This schedule requires fairly continuous sample extraction, exposure, processing and densitometry analysis of step wedges during production runs.
The exposure of the step wedge W of the photosensitive product sample is effected using an optical energy source consistent in wavelength sensitivity with that of the product. For example, typical color photographic negative film is sensitive to 2800 to 4000 color temperature wavelengths provided by a tungsten lamp, graphic arts, whereas "Pagiset", film or paper product is sensitive to infrared wavelengths provided by a solid state laser diode, and some medical radiographic film product is sensitive to visible laser light. For example, a He--Ne gas laser emitting light at 632.8 nm may be used in conjunction with a optical power setting system to successively make the exposures of each step wedge band at successively changed (increased or decreased) power levels to expose a step wedge on a sample of "Pagiset" graphic arts product.
FIGS. 2A and 2B, described in greater detail below, illustrates an earlier system for making the exposure of the test step wedge of FIG. 1, for example. The sample 25 is mounted to a sample transport drum 43 which is rotated by transport control 47 to incrementally advance the sample 25 in the advance direction A of FIG. 1. A laser beam 31 is successively swept by rotation of a polygon mirror 33 in successive line sweeps over the band 1-20 being exposed. Successive lines of each band are thereby exposed as the product sample 25 is incrementally advanced in the advance direction A until the band is fully exposed.
Then, it is necessary to change the laser beam power level to the next step transition. Typically, an acousto-optic modulator (AOM) 27 is employed in the laser beam between the laser 12 and the mirror 33 and product sample 25 to modulate the intensity of the beam 31 exposing each band 1-20 as a function of a step change drive signal provided by a computer 23 from a set of calibrated exposure power values for the particular photosensitive product stored in a standard exposure register 39. The AOM 27 is capable of a high resolution of beam optical power in response to a drive signal, but its response is temperature dependent.
When the AOM 27 is fully OFF, it passes the laser light beam entering it in a direction (not shown) away from the mirror 33 and product sample 25. When the AOM 27 is operated fully ON, it deflects the full intensity laser light beam into the path of light beam 31 and to the polygon mirror 33 (or diverted light beam 31' when flip-in mirror 36 is present). The AOM 27 operates in response to step change drive voltage signals (AOM drive codes) increasing between fully OFF and fully ON by deflecting progressively greater amounts of light in light beam 31 toward the flip-in mirror 33 (or in diverted light beam 31') and lesser amounts of light in the other direction.
The step wedge W of FIG. 1 is preferably composed of twenty bands or steps which are exposed with a 3 Log10 optical power range having a 2 visual density range, resulting in a 0.15 Log10 optical power/step change. As may be inferred from the difficulty in perceiving the density gradations of all of the bands 1-20 of the step wedge W of FIG. 1, single product use, laser sensitometers must be capable of emitted optical power gradations of five to six decades (100,000:1 to 1,000,000:1) of optical power. The accuracy of the sensitometry exposures of the product sample 25 is dependent on the ability to reproducibly provide this wide range of optical power day in and day out. Deviations in the step output power of the laser beam 31 can occur over time due to a number of factors and change the exposure densities in the step wedge bands. If the changes are not detected, the step wedge W used to judge product quality and control the manufacturing process of the product under test will be misinterpreted, leading to inappropriate changes in the manufacturing process and deterioration in product quality. Therefore, it is necessary to calibrate the sensitometer periodically.
In the calibration of the sensitometer, optical power measurements with a minimum resolution of three (one part in 1,000) to six (one part in 1,000,000) significant digits and coverage of five to six decades (100,000:1 to 1,000,000:1) of optical power must be possible. In the sensitometer of FIGS. 2A and 2B, time consuming gain (scale) changes are required to allow the sensitometer electronics to cover that range.
As described below in reference to FIGS. 2A and 2B, because of the time required to fully calibrate the sensitometer in all twenty step wedge optical power levels for each product type, only a limited three step check is typically made prior to each step wedge exposure. If the limited check determines that the sensitometer requires calibration, then a time consuming full calibration is necessary. Collecting optical power measurements of the laser beam during the full calibration has been accomplished using a United Detector Technology model QED-200 quantum efficiency photodiode detector connected to a current-to-voltage converter (CVC) consisting of an operational amplifier and selected feedback resistors. The voltage gain range within the CVC is selected by a computer. Output voltage from the CVC feeds a Keithly Model 617 digital voltmeter (DVM) which auto-ranges to read the output voltage with the best resolution. If the readings taken by the computer from the DVM are outside of the computer analog-to-digital (ADC) conversion range limits, the computer changes the gain of the CVC and initiates another read.
The CVC is also required to drive the output signal 20 feet to the DVM location (within photodiode and power monitoring circuit 10). The use of a current meter in place of a CVC and DVM would involve measuring currents at the fraction of a nanoampere level, well below the resolution limit of even the most precise D'arsonval movement ammeter which are usually accurate to only within a few percent of full scale, with full scale being 1 to 100 microamperes. By comparison, the CVC output voltage is in the range of 0.017 V to 6.0 V, easily measured on a digital voltmeter with substantially less noise sensitivity.
However, there are still several other problems with the above approach. The conversion rate of the DVM is dependent upon the input voltage, the allowed maximum rate decreasing with decreasing voltage level. Because the conversion rate of the DVM is low, each data point consumes up to 750 milliseconds within the DVM at the low end of the voltage range, yielding a sample rate of 1.3 data points per second. The highest DVM conversion rate is 2.9 data points per second at an input voltage of 6.0 V. The relatively low DVM conversion rates adversely impact equipment calibration time.
When a gain change of the CVC is required, the computer reads the CVC output once, changes gain, and then reads the CVC output again. These steps, including DVM latency at low input voltages, can take up to two seconds for one data point reading. Repetition of these readings is necessary in general to eliminate common mode noise due in part to the low input voltage requirement (0.017 volt minimum) to obtain the resolution on the low end of the scales, increasing a single point measurement time to three or four seconds.